1. Field of the Invention
The present invention relates to a method of processing seismic data, in particular multicomponent seismic data, to remove unwanted events from the acquired data. It also relates to an apparatus for processing seismic data. It also relates to a method of and an apparatus for calculating a de-signature and de-multiple operator or a de-signature operator.
2. The Related Art
FIG. 1(a) is a schematic illustration of the principles of a seismic survey. This seismic survey is intended to provide information about a target geological reflector 3 disposed within the earth's interior.
The seismic survey shown in FIG. 1 includes a seismic source 4 disposed on the earth's surface 1. A seismic sensor 5, referred to hereinafter as a “receiver”, is also disposed on the earth's surface, at a distance from the seismic source 4. In use, the seismic source 4 is caused to actuate a pulse of seismic energy, and emitted seismic energy is detected by the receiver 5.
FIG. 1(a) illustrates a land-based seismic survey. However, seismic surveying of this general type is not restricted to land, and may be carried out in a marine environment or in the land-sea transition zone. For example, marine seismic surveying arrangements are known in which one or more seismic sources are towed by a survey vessel; in such an arrangement, the receivers may be disposed on the sea-bed (a so-called “ocean bottom cable” survey or OBC survey), or the receivers may also be towed by a survey vessel. Furthermore, a land-based seismic survey is not limited to the configuration shown in FIG. 1(a), and it is possible for the seismic source or the seismic receiver to be disposed within the earth's surface. For example, in a vertical seismic profile (VSP) seismic survey a seismic source is placed on the earth's surface and a receiver is disposed within the earth's interior in a bore hole. In a reverse VSP survey a seismic source is disposed within a bore hole, and a receiver is disposed on the earth's surface.
Only one seismic source 4 and one seismic receiver 5 are shown in FIG. 1(a) for ease of explanation but, in general, a practical seismic survey will contain an array of sources and an array of receivers.
One problem that arises in seismic surveying is that seismic energy may travel from the source to the receiver along many paths. One reason for this is that many other reflectors exist within the earth in addition to the target reflector. In FIG. 1(a) this is illustrated schematically by a reflector 2 that overlies the target reflector 3. These additional reflectors generate paths of seismic energy from the source to the receiver that involve a reflection at a reflector other than the target reflector. Another reason for the existence of multiple paths of seismic energy is that seismic energy that is propagating upwardly within the earth will undergo reflection at the earth's surface 1, owing to the different seismic properties of the earth and the air. This leads to the existence of paths of seismic energy from the source to the receiver that involve more than a single reflection at the target reflector. These paths give rise to unwanted events in seismic data acquired at the receiver. An event in acquired seismic data that relates to seismic energy that has undergone multiple reflections will hereinafter be referred to a “multiple event”.
FIG. 1(a) illustrates the primary path of seismic energy for this surveying arrangement, in which the path of seismic energy from the source 4 to the receiver 5 involves only a single reflection, at the target reflector 3. (Refraction at the overlying reflector 2 has been omitted from FIG. 1(a) for clarity.) Although seismic energy propagating along the primary path passes through the overlying reflector 2 on its downwards path from the source 4 to the target reflector 3, and again on its upwards path from the target reflector 3 to the receiver 5, the primary path does not involve reflection at the overlying reflector 2. In an ideal seismic survey, only seismic energy that travelled along the primary path would be detected by the receiver 5.
In a practical seismic survey, the receiver 5 will detect seismic energy that has travelled from the source 4 along many paths other than the primary path. Examples of these other paths of seismic energy are shown in FIG. 1(b) to 1(d). FIGS. 1(b) to 1(d) illustrate paths of seismic energy that involve more than one reflection, and these are known as “multiple events”. In FIG. 1(b) downwardly propagating seismic energy from the source 4 is reflected by the overlying reflector 2 so that it travels upwards to the earth's surface 1. The seismic energy is further reflected downwards at the earth's surface, and is then incident on the target reflector. An event of this general type is known as a “source-leg multiple”, since the additional reflections occur in the path of seismic energy from the source to the target reflector.
FIG. 1(c) illustrates a seismic energy path in which the seismic energy travels direct from the source to the target reflector 3, and is reflected upwards at the target reflector 3. However, the reflected seismic energy is not incident direct on the receiver, but is reflected downwards at the earth's surface, and is then reflected upwards at the overlying reflector 2 before reaching the receiver. A seismic path of this type is known as a “receiver-leg multiple”, since the additional reflections occur on the path of seismic energy from the target reflector to the receiver.
FIG. 1(d) illustrates a seismic path in which seismic energy from the source is incident on the target reflector 3, is reflected upwards to the earth's surface, is reflected downwardly and undergoes a further reflection at the target reflector 3 before reaching the receiver. In this seismic energy path, the additional paths occur between the path of seismic energy from the source to the target reflector and the path of energy from the target reflector to the receiver.
The seismic energy acquired at the receiver in a practical seismic survey will include events corresponding to the desired primary path 1(a), but will also contain events relating to unwanted multiple paths such as the paths shown in FIGS. 1(b) to 1(d). In order to provide accurate information about the target reflector it is desirable to be able to identify and remove multiple events from the seismic energy acquired at the receiver.
The seismic sources used in a land-based survey are normally vibrated or explosive sources. If vibrators are used it is possible to perform a multi-component survey, using a multi-component vibrator that produces three orthogonal source motions (two in orthogonal horizontal directions and one in the vertical direction). If a seismic receiver is used that can record particle motion in three orthogonal directions, then it is possible to perform a 3C×3C (or 9C) seismic survey. A suitable receiver for this is one that can measure three orthogonal components of the particle motion at the receiver, for example a receiver that contains three orthogonal geophones—two geophones for measuring two orthogonal horizontal components of the particle motion at the receiver, and a third geophone for measuring the vertical component of the particle motion at the receiver.
A further problem in analysing the results of a seismic survey is that multi-component vibrators, which operate by imposing tractions on the earth's surface, emit three distinct wave types, known as P-, Sv- and Sh-waves (P-waves are pressure waves, and Sv and Sh waves are shear waves). The relative amplitudes of these different wave-types in seismic energy emitted by a multi-component vibrator vary depending on the direction of propagation of the seismic energy. A multi-component receiver records the three wave types, with a sensitivity that depends on the angle of incidence of the received seismic energy. When a geophone measures a component of the wavefield at the earth's surface, both P-waves and the two-types of S-waves are recorded without distinction.
This is illustrated schematically in FIG. 4(a). FIG. 4(a) shows a land-based seismic survey in which a three-component seismic source 4—in this case a multi-component vibrator—and a three-component seismic receiver 5 are disposed on the earth's surface. As indicated in FIG. 4(a), the seismic source 4 emits both P waves and S waves (only one S-wave type is shown for clarity), and the receiver 5 detects both P-waves and S-waves. The four views in FIG. 4(a) show, from left to right:                (i) Seismic energy generated by a horizontal source motion at the vibrator 4, and being received by a horizontally-oriented geophone at the receiver 5;        (ii) Seismic energy being generated by a horizontal source motion at the vibrator 4, and being detected by a vertically-oriented geophone at the receiver 5;        (iii) Seismic energy being generated by a vertical source motion at the vibrator 4 and being detected by a horizontally-oriented geophone at the receiver 5; and        (iv) Seismic energy being generated by a vertical source motion at the vibrator 4 and being detected by a vertically-oriented geophone at the receiver 5.        
FIG. 4(a) illustrates only one horizontal component and one vertical component of the source motion at the vibrator 4 and one horizontal geophone component and one vertical geophone component at the receiver 5. As noted above, in a full multi-component survey the vibrator 4 will further generate seismic energy by a source motion out of the plane of the paper, and the receiver will further comprise a third geophone that detects the particle motion along a line out of the plane of the paper. In total, thus, there are 9 combinations produced by the three orthogonal source motions generated at the source and the three orthogonal geophones at the receiver.
As noted above, many seismic receivers record P- and S-waves without distinction, so that a seismic trace acquired at the receiver will includes events due to received P-waves and events due to received S-waves. In many cases it is desirable to separate the P-events in a seismic trace from the S-events, since this provides additional information about the earth's interior. In many cases, a geological structure will have a different effect on P-waves than on S-waves. The process of separating the P-events in a seismic trace from the S-events is generally referred to as decomposing the seismic trace into its P- and S-components.
There are a number of prior art approaches to decomposing seismic energy acquired at a receiver into P-components and S-components. There are also a number of prior approaches to eliminating the effect of multiple reflections from the acquired seismic data.
For land-seismic data, C. P. A. Wapenaar et al, in “Decomposition of multicomponent seismic data into primary P- and S-wave responses”, Geophys. Prosp. Vol. 38, pp 633-661 (1990) and P. Herrmann, in “Decomposition of multicomponent measurements into P- and S-waves”, Ph.D. thesis, Delft University of Technology (1992) have given an elastic decomposition scheme which decomposes data both at the receiver side (common shot gather) and at the source side (common receiver gather). The wave field decomposition applied to a common source gather along the receiver line replaces the original particle velocity detectors by pure P- and S- wave detectors. The wave field decomposition applied to a common receiver gather similarly replaces the original vibrator sources by pure P- and S-wave sources. Thus, the total decomposition scheme provides 9 different data sets: P, Sv and Sh processed gathers from simulated P, Sv and Sh sources.
After the wave field decomposition, Wapenaar et al (1990, supra) apply an inverse scheme that eliminates the response of the earth's surface from the decomposed results. In this technique, the earth's surface is assumed to be an interface between a solid and a vacuum. Eliminating the response of the earth's surface eliminates all multiple paths that involve reflection at the earth's surface from the decomposed data, so giving the primary P- and S-wave responses of the earth's interior.
This prior technique requires that the seismic sources are point sources and have a known wavelet or have a wavelet that can be estimated from the acquired data. The above prior art technique has been generalised by E. Holvik and L. Amundsen, in “decomposition of multi component sea floor data into primary PP, PS, SP, and SS wave responses”, Expanded Abstracts of 68th Annual Int Mtg of Society of Exploration Geophysicists, pp 2040-2043 (1998), to the case of a marine seismic survey in which ideal vibrators (traction sources) and geophones are deployed at the sea floor. This technique relates to decomposing the acquired data, and removing events arising to reflections within the water layer.
A further prior art technique has been developed by K. Matson and A. Weglein, in “Removing of elastic interface multiples from land and ocean bottom seismic data using inverse scattering” in Expanded Abstracts of 66th Annual Int Mtg of Society of Acceleration of Geophysicists, pp 1526-1529 (1996), and by K. Matson in “An inverse scattering series method for attenuating elastic multiples from multicomponent land and ocean bottom seismic data”, Ph.D. thesis, University of British Columbia (1997). In this technique, inverse scattering series are used to develop elastic schemes that attenuate multiple reflections from multi-component land or ocean bottom seismic data.
The techniques developed by Holvik and Amundsen and by Matson and Weglein again require that the seismic sources must be point sources with a wavelet that is known or that can be estimated from the acquired data.
L. Amundsen has proposed, in “Geophysics” Vol 66 pp 327-341 (2001), a method of eliminating free-surface multiples from marine seismic data acquired using a multicomponent seismic receiver disposed in the water column or on the sea-bed. However, this method does not remove the effects of multiple reflections associated with the sea-bed.